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  1. 1. Plant Photosynthetic Reaction Centers
  2. 2. Photosystem I Each monomer of Photosystem I consists of a dozen proteins and over a hundred cofactors such as (chlorophyll, bright green) and carotenoids (orange).
  3. 3. Photosystem I contains 12 polypeptides, 96 chlorophylls, 2 phylloquinones, three [4Fe-4S] clusters, 22 carotenoids, four lipids and a Ca2+ molecule. PsaA and PsaB (red and blue), PsaF (yellow), PsaL (grey), PsaM (pink) and three stromal proteins [PsaC (magenta), PsaD (blue) and PsaE (cyan)]. Photosystem I exists in the membrane of cyanobacteria as a trimer. Photosystem I: Protein components
  4. 4. PsaA and PsaB heterodimer: location of primary electron transfer chain. peripheral PsaC protein: peripheral, similar to a small, dicluster bacterial ferredoxins. PsaD and PsaE: peripheral, assist in docking ferredoxin, regulate cyclic electron transfer. PsaF: plastocyanin docking. PsaG, PsaH and PsaK: stabilization of the light harvesting complexes. PsaI and PsaJ: structural organization of the PSI complex. PsaL: trimerization of PSI.
  5. 5. Photosystem I: harvesting light These antenna molecules each absorb light and transfer energy to their neighbors. Rapidly, all of the energy funnels into the three reaction centers, where is captured to create activated electrons.
  6. 6. The electron transfer chain The heart of photosystem I is an electron transfer chain, a chain of chlorophyll (green), phylloquinone (orange) and three iron-sulfur clusters (yellow and red). The electron transfer cofactors from P700 to FX are embedded within the membrane phase and thereby shielded from the solvent.
  7. 7. The electron transfer cofactors include a pair of chlorophyll a molecules as the primary electron donor, a chlorophyll a monomer as the primary electron acceptor and a phylloquinone as a secondary electron acceptor. Two molecules of phylloquinone exist per reaction center. The differences with Type II reaction centers exist primarily on the electron acceptor side. Photosystem I utilizes a [4Fe-4S] cluster that, unlike the non-heme iron in the bacterial reaction center, functions in electron transfer. Two additional [4Fe-4S] clusters, termed FA and FB, participate in this process by providing a pathway for electrons to leave the reaction center.
  8. 8. Electron Transfer Rates
  9. 9. Why electrons are transferred to ferredoxin than to plastocyanin?
  10. 10. Ferredoxin The electrons are picked up by the soluble [2Fe-2S] protein, ferredoxin, a one-electron carrier protein, which can in turn form a complex with ferredoxin:NADP+ oxidoreductase to reduce NADP+ to NADPH. Plant-type ferredoxins: 1-8; Halophilic ferredoxins: 9; Vertebrate ferredoxins: 10-11
  11. 11. Ferredoxin-NADP+ Reductase 2 reduced ferredoxin + NADP+ + H+ ↔ 2 oxidized ferredoxin + NADPH NADP-binding site
  12. 12. The transfer of electrons from reduced ferredoxin to NADP+ is catalyzed by ferredoxin- NADP+-reductase. This complex contains a tightly bound FAD which accepts the electrons one at a time from ferredoxin. The FADH2 then transfers a hydride to NADP+ to form NADPH. Ferredoxin is a strong reductant but can only function in one electron reductions. NADP+ can accept two electrons in the form of a hydride. Thus, an intermediary is needed to facilitate the electron transfer.
  13. 13. Glu312 Ser38
  14. 14. Type I and Type II Reaction Centers A summary of the five distinct photosynthetic reaction centers known
  15. 15. Respiratory systems
  16. 16. Photosynthetic systems
  17. 17. Respiration is the major process by which aeorbic organisms derive energy and involving a series of electron carriers resulting in the reduction of dioxygen to water. The inner mitochondrial membrane is involved in energy transduction with protein complexes transferring electrons in steps coupled to the generation of proton gradient. Respiratory complexes In eukaryotes this process is confined to the mitochondrion.
  18. 18. In chloroplasts, light drives the conversion of water to oxygen and NADP+ to NADPH with transfer of H+ ions across chloroplast membranes. In mitochondria, it is the conversion of oxygen to water, NADH to NAD+ and succinate to fumarate that are required to generate the proton gradient. Photosynthesis vs Respiration
  19. 19. Photosynthesis Respiration Production of ATP Yes Yes (~ 30-32 ATP molecules per glucose) Reactants 6CO2 and 12H2O and light energy C6H12O6 and 6O2 Requirement of sunlight Yes No Chemical reaction 6CO2 + 12H2O + light --> C6H12O6 + 6O2 + 6H20 6O2 + C6H12O6 --> 6CO2 +6H2O + energy Process The production of organic carbon (glucose and starch) from inorganic carbon (carbon dioxide) The production of ATP from the oxidation of organic sugar compounds Fate of oxygen and carbon dioxide Carbon dioxide is absorbed and oxygen is released Oxygen is absorbed and carbon dioxide is released What powers ATP synthase H+ gradient across thylakoid membrane into stroma H+ gradient across the inner mitochondria membrane into matrix What pumps protons across the membrane Electron transport chain Electrochemical gradient created energy that the protons use to flow passively synthesizing ATP Final electron receptor NADP+ (forms NADPH ) O2 (Oxygen gas) Organisms Occurs in plants, protista (algae) and some bacteria. Occurs in all living organisms Electron source Oxidation H2O at PSII Glucose, NADH + , FADH2 Catalyst Chlorophyll No catalyst High electron potential energy From light photons From breaking bonds
  20. 20. Mitochondrial redox carriers NADH Complex I Q Complex III Cytochrome C Complex IV O2 Complex II FADH The inner membrane contain four macromolecular complexes that catalyze the oxidation of substrates such NADH/FADH2 through the action of metallo-proteins such as cytochromes and iron sulfur proteins.
  21. 21. Complex I: NADH dehydrogenase In mammals, there are 44 separate polypeptide chains, a FMN and eight iron-sulfur clusters (FeS). The structure of the 536 kDa complex comprises 16 different subunits with 64 transmembrane helices and 9 Fe-S clusters. There 14 ‘core’ subunits highly conserved from bacteria to humans.
  22. 22. Electron transfer mechanism NADH is oxidized to NAD+, by reducing FMN to FMNH2 in one two-electron step. FMNH2 is then oxidized in two one-electron steps, through a semiquinone intermediate.
  23. 23. Each electron thus transfers from the FMNH2 to an Fe-S cluster to ubiquinone (Q). Transfer of the first electron results in the free-radical Q* (semiquinone) and transfer of the second electron reduces the Q* to QH2 (ubiquinol). During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space.
  24. 24. The transfer of two electrons from NADH to oxygen, through complexes I, III (bc1) and IV (cytochrome c oxidase) results in the translocation of 10 protons across the membrane, creating the proton-motive force (pmf) for the synthesis of ATP by ATP synthase. NADH + H+ + CoQ + 4H+ in → NAD+ + CoQH2 + 4H+ out It catalyses the transfer of two electrons from NADH to ubiquinone, coupled to the translocation of four protons across the bacterial or inner mitochondrial membrane Overall reaction Complex I is a reversible machine able to utilize pmf and ubiquinol to reduce NAD+.
  25. 25. Complex II: Succinate dehydrogenase Complex II consists of four protein subunits: SdhA, SdhB, SdhC and SdhD. It is the only enzyme that participates in both the TCA and the ETC chain by catalyzing the oxidation of succinate to fumarate with the reduction of ubiquinone to ubiquinol.
  26. 26. Electron transport
  27. 27. Complex III: Cytochrome bc1 complex Most of the primitive members of this family contain a b-type cytochrome, a c-type cytochrome and an iron sulfur protein (ISP). Cytochrome b Rieske Protein Core 1 Protein Core 2 Protein Matrix side Cytosolic side Transmembrane region Isolated cytochrome bc1 complexes from eukaryotic organisms contain 10/11 subunits including a b-type cytochrome with two heme centres, an iron-sulfur protein (Reiske protein) and a mono heme c-type cytochrome.
  28. 28. The complex oxidizes quinols and transfers electrons to soluble acceptors such as cytochrome c. QH2 + 2 cytochrome c (FeIII) + 2 H+ in → Q + 2 cytochrome c (FeII) + 4 H+ out Electron transport
  29. 29. Complex IV: cytochrome c oxidase Cytochrome c oxidase is the final complex of the respiratory chain catalyzing dioxygen reduction to water. The complex contains several metal prosthetic sites and 14 protein subunits in mammals. Isolation of cytochrome oxidase has two heme groups (a and a3) together with two Cu centers (CuA and CuB).
  30. 30. Electron transport Cyt c  CuA  heme a  heme a3-CuB The overall reaction: 4 Fe2+-cyt c + O2 + 8H+ in  4 Fe3+- Cyt c + 2H2O + 4H+ out